The invention relates to a device for curing an adhesive by means of radiation in an at least substantially oxygen-free atmosphere, which adhesive is interposed as an adhesive layer between two superposed layers of a disc-shaped information carrier, which layers together define a peripheral edge of the information carrier, which device comprises a radiation source and a supply means for a gas, not being oxygen.
Such a device is known and is used in the production of optical information carriers of a type comprising two substrates glued to one another, at least one of said substrates carrying an information layer. During production a UV-curing polymerizable adhesive is applied, after which the assembly thus formed is irradiated by means of a UV irradiation device in a nitrogen atmosphere. The adhesive then cures by polymerization reactions in which radicals are formed.
DE-A 3642961 refers to the use of UV-curing adhesives as alternatives for epoxy-resin adhesives. However, a method in which such adhesives are cured in a nitrogen atmosphere is not considered to be a suitable mass-production method because of the intricate equipment required for this.
It is an object of the invention to provide a device of the type defined in the opening paragraph, enabling adhesive layers to be cured during the large-scale production of information carriers.
To this end, the device in accordance with the invention is characterized by the presence of a holding means for holding the information carrier in a centered position with respect to a centering axis, the gas supply means having discharge openings for a gas, not being oxygen, which discharge openings are situated in a circular zone around the centering axis, at a distance from the centering axis which at least substantially corresponds to the radius of the peripheral edge of the information carrier, so as to cause a gas, not being oxygen, to flow past the peripheral edge during curing. Preferably, the discharge openings for inert gas are situated in a wall portion of the device.
It will be evident that said information carrier is stationary while the device in accordance with the invention is used, i.e. while the adhesive layer is being cured.
At least one of the layers to be attached to one another by means of the adhesive is transparent to the radiation used. In the present context the term xe2x80x9clayerxe2x80x9d is to be understood to mean also a substrate. In principle, the adhesive means can be adhesives such as photo-polymerizable acrylates.
In the device in accordance with the invention an at least substantially oxygen-free atmosphere is created locally, specifically at the peripheral edge of the information carrier, in order to preclude that radicals formed at the periphery during curing are destroyed by reaction with oxygen from the air. It has been found that in this way curing of the entire adhesive layer is possible at a comparatively low irradiation intensity. Another advantage of supplying the gas for the expulsion of the oxygen only at the periphery is that the supplied gas is used efficiently, as a result of which the required amount of gas is comparatively small. A suitable gas is preferably an inert gas. Preference is then given to nitrogen but gases such as argon, helium or neon are also suitable. Preferably, the radiation source is a UV source when conventional adhesives are used.
The device in accordance with the invention can be used successfully in the manufacture of optical discs in accordance with the recently defined DVD standard. Such discs comprise two substrates connected to one another by means of an adhesive layer, at least one of the substrates being provided with an information layer. In certain versions the adhesive layer should have a specified thickness and should be transparent.
A practical embodiment of the device in accordance with the invention is characterized by the presence of a curing chamber having a wall formed with discharge openings, in which chamber the holding means is disposed at least during curing, which chamber has discharge means for gas. For a gas flow directed towards the peripheral edge the discharge openings are preferably situated opposite the holding means, viewed along the centering axis.
In order to guarantee a uniform flow around the information carrier during curing the device in accordance with the invention preferably has the features defined in claim 5. These features further assure a uniform irradiation of the information carrier.
An optimization of the local flow around the information carrier during curing is achieved by means of an embodiment which is characterized in that the discharge openings are situated in a wall portion of the device, which wall portion extends opposite the holding means and which has a groove which surrounds the centering axis, in which groove at least a plurality of the discharge openings are situated. The presence of the groove, particularly in conjunction with a turntable which is rotated during curing, ensures that a highly oxygen-free gas atmosphere is formed, the peripheral edge being constantly disposed in the supplied gas.
An optimization of the irradiation of the information carrier during curing is achieved with an embodiment of the invention having the features defined in claim 7. These features lead to a reduction of the required power of the radiation source.
An embodiment of the device in accordance with the invention, which enables an information carrier to be loaded into and removed from the curing chamber in a simple manner, is characterized by the features as defined in claim 8.
An embodiment of the device in accordance with the invention, which is very suitable for mass production, is characterized in that the holding means forms part of a collection of holding means which can be positioned successively with respect to the discharge openings.
The invention further relates to a method of manufacturing a disc-shaped information carrier having two superposed layers between which an adhesive layer extends. In this connection the invention aims at providing a method which enables an adhesive layer to be cured during the manufacture of an information carrier in mass-production quantities.
The method in accordance with the invention is characterized in that for curing of the adhesive layer use is made of the device in accordance with the invention.
The invention moreover relates to a further method of manufacturing a disc-shaped information carrier having two superposed layers between which an adhesive layer extends. In this connection the invention aims at providing a method which enables a highly homogeneous adhesive layer to be formed.
To this end, the further method in accordance with the invention is characterized in that an adhesive is applied to one of the layers in two phases, a face of the respective layer being wholly covered with an adhesive during a first phase, after which an annular zone of adhesive is formed during a second phase, upon which both layers are moved opposite one another and towards one another and are subsequently pressed onto one another, after which curing of the adhesive is effected. Surprisingly, it has been found that this method of affixing layers to one another, layers being meant to include substrates, makes it possible to form a homogeneous adhesive layer free of gas bubbles. The adhesive is preferably cured by means of the afore-mentioned device in accordance with the invention but the present method is not limited thereto.
During the manufacture of certain information carriers, particularly optical information carriers, such as certain types of DVDs, it is necessary to form a uniform adhesive layer of a well-defined layer thickness, without any gas inclusions, which are generally air inclusions, being formed. In order to obtain an adhesive layer without gas inclusions it has proved to be favorable to feed an ionized gas stream, for example an ionized air stream, past the applied adhesive after completion of the first phase, during which phase one of the layers is actually wetted with the adhesive, but prior to the beginning of the second phase. Satisfactory results have been achieved with a gas stream having negative ions. Upon completion of the second phase the layers are brought into contact with one another as rapidly as possible. It has been found that just before actual contact is established the adhesive present in the zone slightly spreads over the wetted layer owing to a potential difference which prevails between the annular zone of adhesive and the other layer, which has not been provided with adhesive. After contact has been made and during the subsequent pressing-down the adhesive spreads over the entire area of the layers without any gas inclusions being formed. In this respect it has proved to be important that after a first contact has been made the layers are moved slowly towards one another to allow the adhesive to spread over the whole area of the layers. An important further aspect is that upon completion of the second phase the layers extend parallel to one another as they come into contact with one another and are pressed onto one another. In the case of non-flat layers it may be favorable to give one or both layers a slightly curved shape, particularly a convex shape.
In order to obtain an adhesive layer having a well-defined layer thickness, it has proved to be advantageous to provide one or both layers, prior to the adhesion process, with an annular recess for receiving excess adhesive, said recess is preferably an annular recess adjoining the usually present central hole of the information carrier. Such a recess, as well as any excess adhesive, is situated outside the information area of the information carrier thus manufactured and is therefore not a hindrance. Moreover, using this measure has the advantage that the layers adhere to one another over their whole areas, which has a favorable effect on the mechanical stability of the information carrier thus manufactured.
If the layers are spun prior to being pressed together, it is recommended to provide one or both layers with an annular projection adjoining said annular recess. Such a projection gives rise to a capillary action on the adhesive, thereby precluding the outflow of adhesive from the recess during spinning.
Within the scope of the foregoing, the invention also relates to an information carrier as defined in claim 15 or 16.